typescript function parameter infer types between params

ts-function-param-unions.png

ts-function-param-unions.ts

type LineItem = {
    name: string,
    quantity: number,
};

type ObjectEntriesUnion<T extends Record<string, unknown>> = {
    [K in keyof T]-?: { key: K, value: T[K] }
}[keyof T];

type LineItemEntries = ObjectEntriesUnion<LineItem>;
//      ^? type LineItemEntries = { key: "name"; value: string;} | { key: "quantity"; value: number;}

type UpdateAction = { type: 'updateLineItemValue' } & ObjectEntriesUnion<LineItem>;

function objectParam(req: LineItemEntries) {
    if (req.key === 'name') {
        console.log(req.value); // NOTE: CAN infer type here
        //               ^? (property) value: string
    }
}

function separateParams<T extends UpdateAction, K extends T['key']>(reactKey: string, lineItemKey: K, value: Extract<T, { key: K }>['value']) {
    if (lineItemKey === 'name') {
        console.log(value); // NOTE: can't infer type here
        //            ^? (parameter) value: string | number
    }
}

separateParams('', 'quantity', 0);
separateParams('', 'name', '');

objectParam({
    key: 'name',
    value
    // ^? (property) value: string
});

objectParam({
    key: 'quantity',
    value
    // ^? (property) value: number
});